Histidine triad enzymes are a superfamily of relatively small (MW 28-34 Kda), homodimeric nucleoside monophosphate hydrolases and transferases containing an active site motif related to His-X-His-X-His-X-X, where X is a hydrophobic amino acid (Bieganowski, P., et al., J. Biol. Chem., 2002, 277, 10852-10860). The HINT branch is the most ancient branch, having representatives in all forms of life (Bieganowski, P., et al., J. Biol. Chem., 2002, 277, 10852-10860; and Brenner, C., et al., Nature Struc. Biol., 1997, 4, 231-238).
The signature histidine triad residues were shown to be largely responsible for stabilizing binding to the phosphates, while the base appeared to be sandwiched between two phenylalanines and an isoleucine (Brenner, C., et al., Nature Struc. Biol., 1997, 4, 231-238; and Lima, C. D., et al., Science, 1997, 278, 286-290). Further inspection revealed that N-7 and O-4 for guanosine and N-7 and N-4 for adenosine are almost completely solvent exposed, while little, if any, specific binding interactions surrounding N-2 and N-3 were observed. Hydrogen bonding interactions were observed between Asp-43 and both of the 2′- and 3′-hydroxyl groups, which is consistent with the reduced ability of 2′-deoxy nucleoside phosphoramidates to serve as substrates.
Although HINTs have been associated only recently with adenosine phosphoramidase activity, nucleoside phosphoramidase activity has been observed in partially purified extracts from rabbit liver, whole cells and extracts of the human T-leukemia, CEM cells, peripheral blood mononuclear cells (PBMCs) and green monkey Vero cells (Ledneva, R. K., et al., 1970, Dokl. Akad. Nauk Sssr 193, 1308-10; Dudkin, S. M., et al., 1971, Febs 16, 48-50; Abraham, T. W., et al., 1996, J. Med. Chem. 39, 4569-4575; Abraham, T. W., et al., 1997, Nucleosides Nucleotides 16, 2079-2092; Chang, S.-L., et al., 2001, J. Med. Chem. 44, 223-231).
U.S. Pat. No. 6,475,985 reports certain specific nucleoside phosphoramidate analogs having anticancer and/or antiviral properties. There continues to be an interest in phosphoramidate nucleoside analogs due to their demonstrated utility as prodrugs of antiviral and anticancer nucleoside monophosphates, or pronucleotides (McGuigan, C., et al., 1993, Bioorg. Med. Chem. Lett. 3, 1207-1210; Balzarini, J., et al., 1996, Mol. Pharmacol. 50, 1207-1213; Chang, S. L., et al., 2000, Nucleosides, Nucleotides & Nucleic Acids 19, 87-100; Kim, J., Drontle, et al., 2004, Nucleos. Nucleot. Nucleic Acids 23, 483-493; and Klein, M. G., et al., 1998, Exp. Cell. Res. 244, 26-32).
In spite of the above reports, there is currently a need for chemotherapeutic agents with antiviral and or anticancer properties. In particular there is a need for agents that are selectively activated at the sight of a disease or virus.